The invention relates to a refiner for refining fibrous material, the refiner comprising at least one first refiner element and at least one second refiner element, the second refiner element being arranged around the first refiner element in such a manner that the first refiner element and the second refiner element have a common middle axis and that there is a refining space between the first refiner element and the second refiner element and that the first refiner element and/or the second refiner element are arranged to rotate around said middle axis and that the refiner elements comprise refining surfaces with openings, through which the fibrous material to be refined is fed into the refining space or through which the refined fibrous material exits the refining space.
Refiners for treating fibrous material typically comprise two, possibly even more refiner elements substantially opposite to one another, between which there is a refining space or refiner gap to which the fibrous material to be refined is fed. At least one of the refiner elements is arranged to move with respect to the opposite refiner element. The movable refiner element which typically rotates around its axis may also be called the rotor, and the fixed refiner element may also be called the stator. The refiner elements comprise the refining surfaces that carry out the actual refining, whereby the refining surfaces may be one integral structure or they may consist of a plurality of refining surface segments or blade segments arranged adjacent to one another, the refining surfaces of individual refining surface segments forming one uniform refining surface.
The refining space is a space which is formed between the refining surfaces of the rotor and the stator and where the refining takes place. The refining is caused by mutual pressing and motion of the refining surfaces as a result of frictional forces between the refining surfaces and the material to be refined and, on the other hand, due to frictional forces inside the material to be refined. The surface area between the refining surfaces of the rotor and the stator is the refining area, by which the refining between the refining surfaces of the rotor and the stator takes place in the refining space. The shortest distance between the refining surfaces of the rotor and the stator in the region of the refining area is the blade gap.
To increase the production of refiners, it is important to guide the fibrous material to be refined efficiently between the opposite refining surfaces. At the same time, it is naturally important to enable the removal of sufficiently refined material from between the refining surfaces in such a manner that the refined material does not block up the refining space between the refining surfaces and thus weaken the production of the refiner. For instance the refining surfaces, which comprise blade bars and blade grooves in such a manner that the fibrous material is refined between the blade bars of the opposite refining surfaces and both the material to be refined and the already refined material are able to move in the blade grooves between the blade bars on the refining surface, may have special dams on the bottom of the blade grooves. The dams force the material being refined to move away from the bottom of the grooves and on between the opposite refining surfaces. However, the effect of the dams is local and does not substantially benefit the whole area of the refining surface. The dams also diminish the hydraulic capacity of the refining surface considerably.
Publication EP 0597860 B1 discloses a refiner comprising a substantially cylindrical movable refiner element, i.e. a rotor, and stator shoes, i.e. fixed refiner elements, against it, the stator shoes together providing the fixed refining surface for the refiner. Depending on the embodiment of the publication, the fixed refining surface of the refiner is located on the side of either the inner periphery or the outer periphery of the rotor and extends along a part of the rotor in the circumferential direction. Both the rotor and the stator shoes comprise perforations extending through them so that the fibrous material to be refined may be fed via the perforations in the rotor in between the rotor and the stator shoes and that the refined fibrous material may exit from between the rotor and the stator shoes via the perforations in the stator shoes. The refiner according to the publication also comprises special flow guide means, by which fibrous material to be refined is fed in the circumferential direction of the rotor in such a manner that material is fed to the front part of the stator shoes in the rotational direction of the rotor. Through the perforations extending through both the rotor and the stator shoes, it is possible to feed material to be refined in between the rotor and the stator shoes and to remove the refined material quite efficiently therefrom. However, the efficiency of feed of material to be refined in the refiner of the publication is restricted by the fact that material to be refined is fed to a very small area, i.e. only to the front part of the stator shoes.